According to current legislation and statutory regulations, a self-monitoring function (on-board diagnosis) which monitors adherence to the maximum permissible emissions of hydrocarbons, carbon monoxide and nitrogen oxides is specified for new vehicles with an internal combustion engine. In order to comply with the legal requirements, different diagnosis functions are generally integrated within the engine management system of the internal combustion engine. Special importance is attached in this context in particular to the diagnosis of catalytic converters present in the exhaust gas tract of the internal combustion engine.
Methods for the diagnosis of catalytic converters are currently in general use in which the oxygen storage capacity (OSC) of the catalytic converter is determined and used as a measure for the ability of the catalytic converter to convert hydrocarbons, carbon monoxide and nitrogen oxides. The core of OSC-based catalytic converter diagnosis is determining the ability of the catalytic converter to store oxygen. For this purpose a balance is typically kept of oxygen volumes which flow into or, as the case may be, flow out of the catalytic converter in a defined period of time. At the same time it must be ensured by means of suitable measures that the volume of oxygen already stored in the catalytic converter does not give rise to any errors when determining the OSC.
A common feature of all currently known methods for determining the OSC is that they require an exhaust gas probe upstream and an exhaust gas probe downstream of the catalytic converter that is to be diagnosed. If one of these exhaust gas probes is not present, it is not possible to diagnose the catalytic converter on the basis of the oxygen storage capacity. In particular for exhaust gas cleaning systems in a Y configuration, variants can occur in which exhaust gas probes are not provided upstream and downstream of all the catalytic converters which are present. With a first exhaust manifold and a second exhaust manifold, exhaust gas cleaning systems in a Y configuration have two exhaust manifolds, to which a first individual catalytic converter and a second individual catalytic converter are assigned respectively. Downstream of the individual catalytic converters the exhaust gas comes together in a common exhaust pipe. Further downstream the common exhaust pipe opens into a main catalytic converter.
In order to determine the oxygen storage capacity of all three catalytic converters of an exhaust gas cleaning system in Y configuration with the methods which are conventionally used, five exhaust gas probes are needed: one exhaust gas probe upstream of each of the individual catalytic converters and between each of the individual catalytic converters and the main catalytic converter, as well as downstream of the main catalytic converter. For reasons of cost it may be necessary to dispense with one exhaust gas probe between an individual catalytic converter and the main catalytic converter. The oxygen storage capacity of this individual catalytic converter cannot then be determined by means of the methods which are conventionally used.